There are a large number of processes for hydrocracking petroleum hydrocarbon feedstocks and numerous catalysts which are used in these processes. Many of these processes comprise two stages, a feed preparation stage and a hydrocracking stage, with the two stages being operated with different catalysts. The first stage, in general, contains a hydrodenitrification/hydrodesulfurization catalyst which also may include a hydrocracking function for low conversion hydrocracking, and the second stage contains a hydrocracking catalyst. Product from the first stage may be treated to remove ammonia and hydrogen sulfide gases prior to being passed to the second stage, or product may be passed directly to the second stage hydrocracker.
Recent years have witnessed substantial growth in the application of these catalytic hydrocracking processes. Initially, the desired product from such hydrocracking processes was gasoline, and it was found at an early stage that a desirable high quality naphtha product could be obtained by hydrocracking in a hydrogen atmosphere containing hydrogen sulfide. However, increasing demands for non-aromatic hydrocracked products such as jet fuels and diesel fuels have created somewhat of a dilemma in the design and operation of hydrocracking units, particularly where both high quality gasoline and naphtha products and a non-aromatic jet fuel and diesel fuel type products are desired. Hydrogen sulfide-free hydrocracking systems, i.e., "sweet", are desirable for maximum jet fuel quality, whereas the hydrogen sulfide-containing hydrocracking systems, i.e., "sour", are desired for maximum gasoline quality. In cases where the hydrocracking feedstock is free of sulfur, such as by virtue of a separate hydrofining operation, the problem can sometimes be solved by operating alternately sweet and sour, as disclosed in U.S. Pat. No. 3,132,090, issued on May 5, 1964, and U.S. Pat. No. 3,655,551, issued on Apr. 11, 1972.
It has now been found that high quality gasoline, naphtha, jet fuel and diesel fuel products can be simultaneously produced in a two stage hydrocracking process in which the effluent from the first stage hydrocracking zone is split into two streams, with one stream being passed to a second stage hydrocracking zone containing a hydrocracking catalyst and the other stream being passed to an aromatics saturation zone containing an aromatics saturation catalyst. The effluents from both the hydrocracking zone and the aromatics saturation zone are then passed to one or more fractionating zones in which the effluents are separated into a tops fraction from which light gasoline (i.e., a C.sub.5 /C.sub.6 stream with a high isoparaffin to normal paraffin ratio), naphtha (i.e., light naphtha and heavy naphtha), jet fuel and diesel fuel products are recovered, and a bottoms fraction which may be recycled to the second stage hydrocracking zone and/or the aromatics saturation zone.